The selective catalytic reduction (SCR) of nitrogen oxides (NOx), including nitrogen monoxide (also known as nitric oxide (NO)) and nitrogen dioxide (NO2), to dinitrogen (N2) with ammonia (NH3) reductant using a suitable catalyst is well known from treating stationary source exhaust gas (power stations) and is beginning to be used in mobile (e.g. vehicular) applications and its use is set to increase in the near future.
EP 1054722 B1 claims a method of reducing pollutants, including particulates and NOx, in gas streams, comprising passing such gas stream over an oxidation catalyst under conditions effective to convert at least a portion of NO in the gas stream to NO2 and enhance the NO2 content of the gas stream, removing at least a portion of said particulates in a particulate trap, reacting trapped particulate with NO2, adding reductant fluid to the gas stream to form a gas mixture downstream of said trap, and passing the gas mixture over an SCR catalyst under NOx reduction conditions. The disclosure explains that the reductant fluid is suitably ammonia (NH3) but ammonia precursors including urea, ammonium carbamate can also be considered.
EP 1054722 B1 also claims an improved system for treating combustion exhaust gas containing NOx and particulates, comprising in combination and in order, an oxidation catalyst effective to convert at least a portion of NO in said NOx to NO2 and enhance the NO2 content of the exhaust gas, a particulate trap, a source of reductant fluid, injection means for such reductant fluid located downstream of said particulate trap and an SCR catalyst.
EP 0341832 discloses a process for removing, by combustion, particulate deposited on a filter disposed in a diesel exhaust system, wherein exhaust gas containing NO is initially passed without filtering over a catalyst to convert the NO in the exhaust gas to NO2 prior to filtering to remove particulate and wherein the exhaust gas containing NO2 is then used to combust the particulate trapped on the filter, the amount of NO converted to NO2 being sufficient to enable combustion of particulate trapped on filter to proceed at a temperature less than 400° C.